The present invention relates to a stack-type piezoelectric element, a process for producing the same, and a device using said element. In particular, it relates to a stack-type piezoelectric element having a high reliability as a piezoelectric actuator, a process for producing the same, and also to a device using said element.
Since a piezoelectric ceramic generates an electric voltage when a force is applied thereto and produces a displacement or a force when voltage is applied thereto, it is highly useful for use as actuators or sensors. When a piezoelectric ceramic is applied to actuators or sensors, it is usually used in the form of a stacked body thereof with electrodes. Such a stacked body is called a stack-type piezoelectric element.
Stack-type piezoelectric elements are produced, like other ceramic laminates, generally by the green sheet method. A piezoelectric ceramic green sheet is a sheet-formed material obtained by calcining the starting material for piezoelectric ceramic, adding to the resulting powder a suitable binder etc. to form a slurry, and then molding the slurry into a sheet. The green sheet thus obtained is printed with a metal paste by means of screen printing or like methods to form an electrode part. A desired number of thus prepared sheets are stacked, dried, and then sintered at one thousand several hundred degrees (Centigrade) to be converted into ceramic. The product thus obtained is the piezoelectric element stacked body of the prior art. For use as the final product, the piezoelectric device is further subjected to lead formation and resin coating and polarization treatment by application of voltage. Such a process for producing stacked bodies using the conventional green sheet method is disclosed, for example, in Japanese Patent Application Kokai (Laid-open) Nos. 60-41273 (1985), 60-91800 (1985) and 60-229380 (1985). A process flow diagram for prior stack-type piezoelectric device is shown in FIG. 3.
In the above-mentioned prior process shown in FIG. 3 the sintering of green sheets and the baking of electrodes are carried out simultaneously, so that it is seemingly a simple process because of relatively small man-hour required. However, the binder etc. present in green sheets is difficult to remove by heating due to obstruction by electrode layers and, if an additional step of removing the binder by heating is taken, the ceramic body is apt to undergo cracking, and destacking deformation.
Furthermore, since the stacked body of green sheets and electrodes is sintered at a high temperature and at one time, and since the contraction of ceramic during sintering is large, a large strain (in other words, internal stress) develops inside the piezoelectric ceramic stacked body. Accordingly, when a high voltage is applied to the stacked body or when it is used repeatedly for a long period, the stacked body undergoes mechanical breakage.
Moreover, a large strain develops also in the polarization treatment conducted after sintering, which also leads to lowering of reliability of strength.
Thus, the stack-type piezoelectric element of the prior art is itself poor in reliability of strength and hence not fully satisfactory for use as actuators etc.
Further, the electrode provided between piezoelectric ceramics, has been conventionally made of silver or silver-palladium (Ag-Pd) alloy. This is because, since the sintering of green sheets of piezoelectric ceramic and the stacking thereof with electrodes are simultaneously effected at about 1300.degree. C., a noble metal stable at high temperature must be used as the electrode material. However, this gives rise to the following problem. That is, water can penetrate into the element through a molding resin protecting the side face of the piezoelectric element, pass through between electrodes, and further form an aqueous layer between the stacked body and the resin. Then, silver is dissolved out as ions (Ag+), which are then attracted by an electric field to accumulate in the vicinity of a neighboring electrode, forming a current-carrying path. In consequence, short circuit of electrodes occurs in the prior art piezoelectric element and device when it is operated in a humid atmosphere. The above-mentioned phenomenon of current-carrying path formation is generally called "migration" and is a serious problem common to electrical parts using electrodes made of silver or silver alloy.